1. Field of the Invention
The present invention relates to a method for the removal of mercury from a stream of flue gas obtained from the combustion of coal. The method includes the step of introducing a kaolin or metakaolin-containing sorbent into the stream of flue gas for removing mercury from the stream of flue gas and contacting the mercury with the kaolin or metakaolin-containing sorbent.
2. Description of the Related Art
Coal contains mercury. When the coal is combusted to generate heat and/or electricity, the mercury is released. This is undesirable from an environmental point of view. For this reason, mercury is removed using a sorbent. Metakaolin-containing sorbents have been found satisfactory for this purpose. Removal of mercury using a metakaolin-containing sorbent is disclosed in W002/058823.
Further the applicant has presented results of the removal of mercury from a flue gas at a temperature of over 900° C. and even over 1000° C. at a conference in 2007. The presentation has been published (“Mercury Capture and Fixation in Coal Fired Power Plants using MinPlus Sorbent”, Joep J. P. Biermann and Jost O. L. Wendt, Air Quality VI conference, Arlington Va., September 2007). The flue gas was obtained by combusting bituminous coal, i.e. a coal type having an ASTM D388 coal rank of <3. Accordingly this presentation taught a method for the removal of mercury from a stream of flue gas obtained from the combustion of at least one coal type having an ASTM D388 coal rank of <3. The method includes the step of introducing a kaolin or metakaolin-containing sorbent into the stream of flue gas for removing mercury from the stream of flue gas and contacting the mercury with the kaolin or metakaolin-containing sorbent at a point where both the kaolin or metakaolin-containing sorbent and active free silica (due to the combustion of the coal) are present in the stream of flue gas. The temperature of the flue gas is at least 900° C.
Response on this presentation indicated that the amount of sorbent to achieve the result presented at the conference was deemed too large. This use of large amounts of sorbent increases the cost of removal of mercury, and not just because of the cost of the sorbent. The cost is also increased because spent sorbent has to be removed from the flue gas, e.g. using an electrostatic filter or a bag filter, and if more spent sorbent is to be removed from the stream of flue gas, both the financial outlay for filters and the cost of operating them is increased. In addition or alternatively, the mercury can be removed from the flue gas at relatively lower temperatures, i.e. after it has cooled down more. This also increases the cost of a facility and the operation thereof, because in this low-temperature case carbon-containing sorbents have to be used. These carbon-containing sorbents generally reduce the sale-potential of the fly-ash and spent sorbent mixture collected from the filters. Moreover, these carbon-containing sorbents are more difficult to handle and may cause fires in downstream equipment.
What is needed in the art is to provide an improved method for the removal of mercury from a stream of flue gas obtained from the combustion of coal.